Proteinaceous nanopores have been studied for the past decade for their essential role in biology as nanoscale channels regulating the ion flow through cell membranes as well as exhibiting ion selectivity. Properties of the track-etched membranes have been studied in comparison to the properties of the various biological channels. In the past few years, artificial nanopores in dielectric membranes etched by high-energy ion or electron beams [1,2] have been proposed as a substitute to biological ion channels [3-5]. However, such membranes are electrically insulating and do not provide tunable electrostatic control of the ion concentration inside or the ion flow through the nanopore. Recently, Karnik et al. [6] experimentally demonstrated the metallic gate-voltage modulation of ions and molecules concentration in a long channel with a nanoscale diameter to control the ionic conductance. Gold nanotubes with fully controlled ionic selectivity were reported in ref [7]. The ion selectivity was controlled by applying voltage to the tubes. Also, it was suggested that nanopores in n+-Si membrane can be used as an ion filter by applying a voltage difference between the semiconductor and the electrolyte [8].
Similar to voltage-gated ion channels that belong to a class of transmembrane ion channels activated by changes in the electrical potential difference near the channel, the presence of a surface charge in a solid-state membrane is central for the use of nanopores in single-molecule detection, ion/protein filtering [8], and potentially in DNA sequencing [9-11]. While the surface charge of biological channels can be positive, negative, or spatially distributed in the pore to operate the “gating” mechanism interrupting the flow of molecules, water or ions, the surface charge in solid-state nanopores is usually negative and results from the fabrication process [9]. In this context, conical nanopores in polymer membranes with various (negative) surface charges have been investigated as ion rectifiers [12]. Meanwhile, a microfluidic field effect transistor operating by surface charge modulation in an ion channel has been proposed [13], and theoretical modeling of ion transport in a nanofluidic diode and a bipolar transistor has been developed [14].
There is versatility in the use of semiconductor membranes in controlling the electrolyte charge in a nanopore [15]: unlike dielectric membranes that exhibit negative surface charges inducing positive ion charges at the nanopore surface, n-doped semiconductor membranes can attract either positive or negative ions at the nanopore surface depending on the amount of positive dopant charge in the depletion layer of the n-type semiconductor. Moreover, the semiconductor membrane can be connected to a voltage source to modulate the nanopore channel charge.